C H A P T E R 5
Mathematics and dosage calculations
57
Use the following formula to determine how many
drops of fluid to administer per minute:
drops per minute
(rate)
=
volume
×
drop factor (drops/mL)
time (hours)
×
60
That is, the number of drops per minute, or the rate
that you will set by adjusting the roller clamp on the
IV tubing, is equal to the amount of solution that has
been prescribed per hour times the number of drops
delivered per millilitre (mL), divided by 60 minutes in
an hour.
Try this example: An order has been written for a
person to receive 400 mL of 5% dextrose in water over a
period of 4 hours in a standard system (i.e. 20 drops/mL).
Calculate the correct setting (drops per minute):
rate
=
volume
×
drop factor (drops/mL)
time (hours)
×
60
=
400
4
×
20
60
Simplify:
rate
=
volume
×
drop factor (drops/mL)
time (hours)
×
60
=
400
412
= 33.3
= 33 drops/min
(Note: drops must be in whole numbers as it is not
possible to deliver part
of a drop)
Now calculate the same order for an IV set that
delivers 60 drops/mL:
rate
=
volume
×
drop factor (drops/mL)
time (hours)
×
60
=
400
4
×
60
60
= 100 drops/min
If a person has an order for an IV drug, the same
principle can be used to calculate the speed of the
delivery. For example, an order is written for a person to
receive 60 mL of an antibiotic over 30 minutes. The IV
set used dispenses 20 drops/mL, which allows greater
control. Calculate how fast the delivery should be:
rate
=
volume
×
drop factor (drops/mL)
time (hours)
×
60
=
60
0.5
×
20
60
=
1200
30
= 40 drops/min
Paediatric considerations
For most drugs, children require doses different to those
given to adults. The “standard” drug dose that is listed
on package inserts and in many references refers to
the dose that has been found to be most effective in a
70 kg adult male. An adult’s body handles drugs differ-
ently and may respond to drugs differently to a child’s.
A child’s body may handle a drug differently in all areas
of pharmacokinetics—absorption, distribution, metab-
olism and excretion. The responses of the child’s organs
to the effects of the drug also may vary because of imma-
turity of the organs. Most of the time a child requires a
smaller dose of a drug to achieve the comparable critical
concentration. On rare occasions, a child may require a
higher dose of a drug.
For ethical reasons, drug research is not done on
children. Over time, however, enough information can
be accumulated from experience with the drug to have a
recommended paediatric dose. The drug guide that you
select to use in the clinical setting will have the paediatric
dose listed if this information is available. Unfortu-
nately, there may be times when no recommended dose
for a child is available but that particular drug is needed.
In these situations, established formulae can be used to
estimate the appropriate dose. Determining a paediatric
dose takes into consideration the child’s weight, or body
surface. The
nomogram
that uses body surface area is
more accurate for determining doses (see Figure 5.1).
Regardless of the calculation method used for
children, even a tiny dose error can be critical. When
working in paediatrics, one needs to be familiar with at
least one of these methods of determining the drug dose.
Many institutions require that two nurses check critical
Height
cm
SA
Weight
m
2
lb kg
in.
240
2.0
180 80
70
60
50
40
30
25
20
15
10
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.5
2.0
1.5
1.0
160
140
120
100
90
80
70
60
50
45
40
135
30
25
20
18
16
14
12
10
9
8
7
6
5
4
3
1.5
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
90
80
75
70
65
60
55
50
45
40
35
30
28
26
24
22
20
19
18
17
16
15
14
13
12
220
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
Nomogram
FIGURE 5.1
The West nomogram for calculating body surface area
(BSA). Draw a straight line connecting the child’s height (left scale)
to the child’s weight (right scale). The BSA value, which is calculated
in square metres, is found at the point where the line intersects the
SA column.
